pp60c-src is the cellular homolog of pp60v-src, the transforming protein of Rous Sarcoma Virus. The protein is located on the cytoplasmic face of the plasma membrane and is believed to function in signal transduction in response to mitogens and other extracellular signals including PDGF, NGF and, perhaps, CSF-1. The experiments proposed here make use of recently developed recombinant DNA techniques and address two different, but related, questions concerning the molecular biology of pp60c-src. The first set of experiments examines structure-function relationships in the protein. Eukaryotic viral vectors will be used to obtain milligram quantities of the protein. The purified pp60c-src should facilitate a number of experiments - we propose using it first to prepare monoclonal antibodies and to search for kinases and phosphatases believed to modify the activity of the molecule as a tyrosine kinase. To complement these biochemical studies we will construct a large library of mutant c-src genes using a recombinant murine retrovirus which we have recently constructed and the so-called GC clamp technique from the Manjatis laboratory. The second set of experiments are designed to follow up our recent finding that the level of pp60c-src is modulated some 20-fold during the differentiation of monocytes. We have designed experiments to discover if specific receptors are interacting with pp60c-src in the fully differentiated cells and to see if pp60c-src might also play a role in the differentiation process itself.